Firefighters struggled with heat produced by the fires and the hot, humid weather, officials also said.

The fires “wore them out pretty quick,” Deputy Fire Chief Daniel Kennedy said. “Fighting fires in hot weather is very fatiguing, especially when it’s humid. It takes a lot out of you.”

The first of the alarms was called in at 3:02 p.m., Kennedy said. A multifamily home at 42 Andover St. had an outdated electrical system that had overheated, charring some insulation.

“They were opening up walls and ceilings and chasing it around, but they never found any actual flames,” Kennedy said. The “possibility of a fire was there, but they got it early enough.”

The firefighters shut the building’s power down and had the situation under control by 4:35 p.m.

Kennedy did not know how many people were displaced on Andover Street.

Less than an hour later, firefighters responded to a fire at 55 Cambridge St. That blaze began on the second floor of the two-family home, Kennedy said.

Lawrence was aided by the Lowell, Andover, and North Andover fire departments. One Lawrence firefighter suffered a shoulder injury and was taken to a hospital.

Kennedy did not know the extent of his injuries. Firefighters left the scene by 6:15 p.m. Kennedy did not know how many people were displaced from the Cambridge Street blaze. The cause is still under investigation.

At 8:13 p.m., firefighters received a third call, this time for a fire in a three-story, 30-unit apartment building at 12 Diamond St.

The fire was contained to the third-floor apartment where it started, but displaced 10 residents, fire officials said. The cause was not determined.

An emergency shelter was set up at the Frost Elementary School on Essex Street for residents displaced in the three incidents Wednesday. Kennedy said 20 people are currently being housed there.

“It’s been a busy month,” Kennedy said. “I think this is our third day with multiple fires in July.”